The isolated perfused rat hindquarter is being used for the study of metabolism of the branched-chain amino acids by muscle. Production of carbon dioxide and branched-chain alpha-keto acids from 14C-labelled leucine and valine are being quantified during the course of perfusion of this preparation with glucose and quantities of amino acids in the range found in rat blood. Effects of starvation and of addition of insulin to the perfusion on both transamination and oxidation of the branched-chain amino acids are being examined. Both 1-14C- and U-14C-labelled amino acids are being used in an effort to determine whether metabolism in muscle involves primarily 1-decarboxylation or complete oxidation of the carbon skeleton.